1. Field of the Invention
The present invention generally relates to a light source unit which irradiates laser beams, a phase type optical element which is used in the light source unit, a laser beam scanning device which scans a surface to be scanned with the laser beams irradiated from the light source unit, an image forming apparatus using the laser beam scanning device, and an optical pickup device using the light source unit.
2. Description of the Related Art
Recently, a laser beam emitted from a laser such as a semiconductor laser has been widely used in optical devices such as an optical pickup device, a laser processing device, a laser beam scanning device which is used in, for example, a copying apparatus and a laser printer. In the optical devices, in many cases, the laser beams are used to form a beam spot and the size of the beam spot is preferably as small as possible.
In addition, it is preferable that an error in the manufacturing process of the optical device be small and a malfunction of the optical device caused by an environmental change be prevented. In order to achieve the above, when the laser beams are condensed by a lens, it is preferable that a range of the lens be as wide as possible in the optical axis direction so that the size of the beam spot is less than an allowable beam spot diameter. In the present invention, the range is called “depth margin”. When a using wavelength is λ, a relationship between the depth margin “d” and the beam spot diameter “w” is shown in Formula (1).d∝w2/λ  Formula (1)
That is, when the beam spot diameter “w” is determined at the focal position, the depth margin “d” is also determined.
In addition, from Formula (1), the small beam spot diameter and the wide depth margin conflict with each other. Therefore, the optical device must be designed by balancing the beam spot diameter “w” with the depth margin “d”. When the beam spot diameter “w” is large, the depth margin “d” becomes wide. However, the performance of the optical device is degraded.
As a method which satisfies both the wide depth margin “d” and the small beam spot diameter “w”, a Bessel beam method discovered by J. Durnin can be used. In the Bessel beam method, the amplitude distribution of the beam cross-sections is determined by the first kind zeroth-order Bessel function, and when Bessel beams are transmitted, the amplitude distribution of the beam cross-sections is hardly changed. That is, the Bessel beam is a non-diffraction beam. When it is desired that an exact Bessel beam is generated, an infinitely wide plane wave is required; consequently, it is impossible to generate the exact Bessel beam. However, methods for generating a pseudo Bessel beam are disclosed. In Patent Document 1, an optical scanner is disclosed; in Patent Document 2, a long-focus laser beam generation apparatus is disclosed; in Patent Document 3, a long-focus depth high-resolution irradiating optical system is disclosed; and in Patent Document 4, a Bessel beam generating method is disclosed. In Patent Documents 1 and 2, a method using a ring-shaped slit is disclosed. In Patent Document 3, a method using an axicon prism is disclosed. In Patent Document 4, a method using a binary optical element equivalent to an axicon prism is disclosed.
First, the Bessel beams generated by using the ring-shaped slit are described. FIG. 29 is a diagram showing the Bessel beams generated by using the ring-shaped slit. In FIG. 29, (a) shows an optical system, and (b) through (d) show results by a simulation. In the optical system, a uniform intensity plane wave is input, the plane wave is further input to a ring-shaped slit, and an image is formed by a (perfect) lens having a focal distance of “f”. The ring-shaped slit is disposed at the front focal position of the lens.
Parameters used in the simulation are described. In the ring-shaped slit, the inner diameter φ is 900 μm and the outer diameter φ is 930 μm. The focal distance “f” of the lens is 50 mm, and the wavelength of the light source is 632.8 nm.
In FIG. 29(b), a beam intensity profile of Bessel beams at the focal position (focal plane) of the lens (50 mm from the lens surface) is shown. The beam intensity is standardized so that the peak intensity of the main lobe laser beams is “1”. As shown in FIG. 29(b), the peak intensity of the first-order side lobe laser beams is 13.1% of the peak intensity of the main lobe laser beams, and the peak intensity of the second-order side lobe laser beams is 9.8% of the peak intensity of the main lobe laser beams.
In FIG. 29(c), a relationship between the distance from the lens surface and the beam spot diameter is shown. The beam spot diameter is defined as a diameter of a region where the intensity is 1/e2 or more when the center beam intensity is “1”. The beam spot diameter is hardly changed even if the laser beams are apart from the focal position; that is, it is understandable that the laser beams are non-diffraction beams. In FIG. 29(d), a two-dimensional image generated by the Bessel beams is shown at the focal position.
In addition, in Patent Document 5, an optical scanner is disclosed in which a method for widening a depth margin is shown.
However, when the ring-shaped slit is used, the center part having a large amount of the laser beams is blocked and the light use efficiency becomes very low.
In addition, when the axicon prism or the binary optical element equivalent to the axicon prism is used, as shown in FIG. 30, since the laser beams are not blocked, the light use efficiency is high. However, since the Bessel beams are generated near optical components, suitable arrangement of the optical components is difficult. For example, when the Bessel beams are used in a laser beam scanning device which is used in an image forming apparatus, an optical system such as a relay optical system is newly required so as to conjugate the Bessel region with a non-scanning surface. Consequently, the number of lenses is increased and the optical system becomes large and the cost is increased. In addition, highly precise positioning of the components is required for the optical axis of the optical system. Therefore, an error may occur in the manufacturing process of the optical device and a malfunction of the optical device caused by an environmental change may occur. FIG. 30 is a diagram showing a part of the optical system which generates the Bessel beams by using the axicon prism.
As shown in FIG. 29(b), the Bessel beams include many side lobe laser beams and the beam intensity of the high-order side lobe laser beams is high. When it is assumed that an ideal amplitude distribution of the Bessel function (of the first kind zeroth-order) is obtained, as described above, the peak intensity of the first-order side lobe laser beams is approximately 13% of the peak intensity of the main lobe laser beams and the peak intensity of the second-order side lobe laser beams is approximately 10% of the peak intensity of the main lobe laser beams. That is, the peak intensity of the side lobe laser beams is high. When the side lobe laser beams are generated, the light amount of the main lobe laser beams is decreased. Especially, an area occupied by the high-order side lobe laser beams is greater than that by the low-order side lobe laser beams; therefore, when the peak intensity of the high-order side lobe laser beams is high, the light amount of the main lobe laser beams is greatly decreased. Since the main lobe laser beams are used as an optical signal, the light use efficiency is actually lowered.
In addition, when the Bessel beams are used in an optical device, since the side lobe laser beams are noise laser beams, too large side lobe laser beams may degrade output image quality of the optical device. Especially, the high-order side lobe laser beams are generated at positions apart from the optical axis and are spatially separated from the main lobe laser beams (highest intensity laser beams); therefore, the noise may affect the output image quality of the optical device. For example, when the Bessel beams are used in an optical device of an image forming apparatus and the peak intensity of the side lobe laser beams is at a position apart from the optical axis, a thin line may be formed to frame a dot formed by the main lobe laser beams. This phenomenon also occurs in a laser processing device.
Next, a case is studied in which the method disclosed in Patent Document 5 is applied to an optical system which uses Gaussian beams as input laser beams. In the Gaussian beams, the beam intensity is high at the optical axis and is lowered at a position apart from the optical axis, so that the Gaussian beams are generally used as the laser beams. Therefore, when the Gaussian beams are transmitted through an aperture, the beam intensity at the four corners of the aperture is lower than that at the center of the aperture. In order to make high the beam intensity at the four corners, since a part of the laser beams in the center must be moved to the four corners, high-order laser beams may be generated and the light use efficiency may be lowered. When only the center part of the Gaussian beams is used, the generation of the high-order side lobe laser beams can be avoided; however, since the amount of the blocked laser beams is increased, the light use efficiency may be lowered.
The inventors of the present invention repeated several experiments (simulations), from which the following result is obtained. In a case where laser beams are condensed by a lens, when a phase distribution of the laser beams which are input to the lens is modulated so that the peak intensity of the side lobe laser beams in the beam intensity profile at the focal position of the lens is slightly increased, the beam spot diameter is prevented from being enlarged at a position apart from the focal position in the optical axis direction of the lens.
In addition to widening the depth margin and not to enlarging the beam spot diameter, a miniaturized digital copying apparatus and a miniaturized laser printer both in monochrome and color have been required. That is, a miniaturized laser beam scanning device used in the digital copying apparatus and the laser printer has been required. Especially, a miniaturized color digital copying apparatus and a miniaturized color laser printer, which have large potential in the market, have been required. When the above apparatuses are miniaturized, the amount of the materials used can be decreased and environmentally friendly products can be realized.
In the color digital copying apparatus and the color laser printer, a tandem type has been mainly developed in which four image forming units such as photoconductor bodies corresponding to four colors are used. A laser beam scanning device has been widely used in which four laser beam scanning devices corresponding to the four image forming units are disposed in one housing.
In addition, in the laser beam scanning device, in many cases, two scanning lenses are used, an optical path is folded by an optical path folding mirror, and the two scanning lenses are contained in the housing. However, one of the two scanning lenses behind the optical path folding mirror obstructs the miniaturization of the laser beam scanning device.
When the scanning lens is disposed behind the optical path folding mirror, a big limitation occurs in arranging the optical path folding mirror in the housing and this limitation results in not being able to miniaturize (to decrease the height of) the laser beam scanning device.
In a case where only one scanning lens is used, or even if plural scanning lenses are used, when the one or more scanning lenses are disposed at the side of a light deflector, the limitation in the arrangement can be solved. However, since the magnification of the scanning lens in the sub scanning direction becomes large, the tolerance of the laser beam scanning device becomes large and an environmental change affects the laser beam scanning device. Consequently, the optical system in the laser beam scanning device becomes unstable with the passage of time. Especially, in a digital copying apparatus and a laser printer capable of processing an A3 size (297 mm×420 mm) sheet, since the range to be scanned becomes large and the distance from the light deflector to a surface to be scanned becomes long, the instability becomes remarkably large with the passage of time.
In addition to the above problems, recently, in a laser beam scanning device which is used in image forming apparatuses such as a digital copying apparatus and a laser printer, in order to prevent displacement of an image on a surface to be formed which displacement is caused by an environmental change, a diffraction lens is used (for example, in Patent Documents 6 through 8).
In Patent Document 6, a laser beam scanning device which is used in a digital copying apparatus, a laser printer, a laser facsimile, and so on is disclosed. In the laser beam scanning device, for a lens whose precise shape is maintained in an optical system, the displacement of the focal position of an image caused by a temperature change is prevented. With this, a low-cost and high-performance laser beam scanning device is realized. The laser beam scanning device provides a light source formed of a semiconductor laser, a coupling optical system which couples laser beams from the light source, a first optical system which makes the laser beams from the coupling optical system parallel laser beams in the main scanning direction and makes the laser beams converge on a deflector in the sub scanning direction, where the deflector deflects the laser beams from the first optical system in the main scanning direction, and a scanning optical system for condensing the laser beams deflected by the deflector. The material of all lenses in the coupling optical system is resin and a diffraction optical surface is formed on at least one surface of the lens.
In Patent Document 7, a low-cost laser beam scanning device whose performance is stable during a temperature change is disclosed. The laser beam scanning device provides a light source for emitting laser beams, a deflector for deflecting input laser beams in the main scanning direction, a light source optical system which makes the laser beams from the light source parallel laser beams in the main scanning direction and makes the laser beams condense near the deflection surface of the deflector in the sub scanning direction, and a scanning optical system for condensing again the laser beams deflected by the deflector. The light source optical system is formed of one optical element made of resin, and the optical element provides at least one reflection surface not having a rotationally symmetric axis and two transmission surfaces.
In Patent Document 8, a compact laser beam scanning device suitable for high-precision printing is disclosed that is tolerant of an ambient temperature change and a wavelength change of a semiconductor laser is disclosed. The laser beam scanning device provides a light source for emitting laser beams, an optical system for guiding the laser beams from the light source to a deflector, and an image forming system for guiding the laser beams from the deflector to a surface where an image is formed. The laser beams scan the surface where the image is formed by rotation of the deflector. The optical system provides a diffraction section on one or more surfaces of the optical system and satisfies a predetermined conditional formula.
In addition, in an optical pickup system, a diffraction optical element is used to achieve compatibility in plural wavelengths (for example, of red, blue, and colors other than the red and blue) and to correct aberration by a single objective lens (for example, in Patent Documents 9 and 10).
In Patent Document 9, a condensing optical element and an optical pickup device which can reduce the degradation of a coma aberration caused by tracking while achieving compatibility in three wavelengths are disclosed. In the condensing optical element which is used in the optical pickup system which executes reproducing/recording information by using laser beams of wavelengths λ1 through λ3 for first through third optical disks having protective substrate thicknesses of t1 through t3, at least one optical surface is divided into plural concentric circular areas by making the optical axis the center, when the optical system magnification is determined to be “m3” in reproducing/recording information by a finite common benefit system for the third optical disk, the focal distance “f3” of the condensing optical element for the wavelength “λ3” satisfies Formula (2).0.01<|m3|×(t3−t1)/f3<0.07  Formula (2)
In Patent Document 10, a compound objective lens is disclosed. By using the compound objective lens formed of a hologram and an objective lens, stable and high definition compatible reproducing/recording of information is realized in a BD (blue-ray disk) whose substrate thickness is approximately 0.1 mm corresponding to blue laser beams and a DVD (digital versatile disk) whose substrate thickness is approximately 0.6 mm corresponding to red laser beams. The compound objective lens is formed of a hologram and a refraction type lens. The hologram provides a lattice having step-shaped cross sections formed at least at a part of the hologram, where the step difference between the step-shaped cross sections is a value in which a unit step difference “d1” is multiplied by an integer, and the unit step difference “d1” gives an optical path difference of approximately one wavelength for a first laser beam having a wavelength λ1 within a range of 390 nm to 415 nm. One cycle of the lattice is formed of steps having a height 0 times, two times, one time, and three times the unit step difference “d1” in this order in the direction from the optical axis to the outer side of the hologram.
However, in order to obtain a high-quality image from the image forming apparatus such as a digital copying apparatus and a laser printer, the beam spot diameter must be stable. When a diffraction lens as described above is used, the beam spot diameter becomes stable during a temperature change, and the image quality can be improved. The diffraction lens can correct only influence caused by the temperature change; and a change of the beam spot diameter caused by vibration and deformation of the image forming apparatus cannot be corrected.
In order to further stabilize the beam spot diameter, the depth margin must be wider. That is, it is necessary that the beam spot diameter be made not too much wider than usual.
As described above, the relationship between the depth margin and the beam spot diameter is shown in Formula (1). In the diffraction optical element for achieving compatibility in plural wavelengths (for example, of red, blue, and colors other than the red and blue) and for correcting aberration by a single objective lens, Formula (1) is satisfied for the depth margin.
Recently, the beam spot diameter has been smaller than before due to the progress of high density recording and the depth margin is likely to be narrow. With this, the performance may be decreased and the cost may be increased.    [Patent Document 1] Japanese Patent No. 3507244 (Japanese Laid-Open Patent Application No. 9-243945)    [Patent Document 2] Japanese Laid-Open Patent Application No. 9-064444    [Patent Document 3] Japanese Laid-Open Patent Application No. 4-171415    [Patent Document 4] Japanese Laid-Open Patent Application No. 10-227992    [Patent Document 5] Japanese Laid-Open Patent Application No. 2000-249950    [Patent Document 6] Japanese Laid-Open Patent Application No. 2005-258392    [Patent Document 7] Japanese Laid-Open Patent Application No. 2002-287062    [Patent Document 8] Japanese Laid-Open Patent Application No. 2004-126192    [Patent Document 9] Japanese Laid-Open Patent Application No. 2006-012218    [Patent Document 10] Japanese Laid-Open Patent Application No. 2005-129227